Recently, new energy technology has become highlighted owing to environmental problems, and as one of the new energy technology, a fuel cell has been specifically noted. The fuel cell converts chemical energy to electric energy through electrochemical reaction of hydrogen and oxygen, attaining high energy utilization efficiency. Therefore, extensive studies have been carried out on realization of fuel cells for civil use, industrial use, automobile use, etc.
As categorized in accordance with the type of the electrolyte employed therein, fuel cells are known to include phosphate-type, molten carbonate-type, solid oxide-type and solid polymer-type ones and others. With regard to the hydrogen sources for producing hydrogen for these fuel cells, studies have been conducted on liquefied natural gas predominantly containing methane; city gas predominantly containing natural gas; synthetic liquid fuels produced from natural gas; petroleum-derived liquefied petroleum gas; and petroleum-derived hydrocarbons such as naphtha and kerosene. For producing hydrogen from these gaseous or liquid hydrocarbons, in general, the hydrocarbons are, after processed for desulfurization, reformed in a mode of partial oxidation reforming, autothermal reforming, steam reforming or the like in the presence of a reforming catalyst.
The above reformation treatment gives mainly hydrogen and carbon monoxide, of which carbon monoxide may be converted into hydrogen and carbon dioxide through aqueous gas shift reaction with water. The aqueous gas shift reaction is utilized also for changing the ratio of hydrogen and carbon monoxide in the aqueous gas to a desired one in accordance with the object of the production reaction, and it is also applicable to hydrogen production. The copper-zinc-aluminum catalyst for use in the aqueous gas shift reaction is active at a relatively low temperature, as compared with a noble metal-based catalyst, and, therefore, the carbon monoxide concentration may be lowered to a low concentration of at most 1%, which, however, is problematic in that the catalyst may be inactivated owing to copper sintering to occur under heat and steam. Accordingly, the catalyst may be used for a long period of time in an industrial plant that is driven under a constant condition, but in case where stop and start are frequently repeated and the catalyst is repeatedly exposed to oxidation/reduction atmospheres like in a fuel cell, copper sintering may readily occur and the catalyst may be thereby readily inactivated. A catalyst with a noble metal such as platinum supported by titania or ceria has high durability, but its activity at low temperatures is not comparable to that of the copper-zinc-aluminum catalyst.
In that situation, for improving the activity and the durability of the copper-zinc-aluminum catalyst, various investigations have been made, and the following reports have been given.
Alumina or an alumina precursor is previously introduced into a reaction system, and copper and zinc are deposited around the alumina or alumina precursor serving as a nucleus; and the catalyst thus produced has excellent activity and durability (Patent Reference 1). A catalyst comprising, as the indispensable ingredients, copper oxide, zinc oxide and aluminum oxide, and in addition to these, a specific amount of zirconium oxide and manganese oxide has high activity (Patent Reference 2). A catalyst produced from a catalyst precursor that contains both aluminium having a morphology of hydrotalcite and aluminum other than hydrotalcite is highly active (Patent Reference 3). A catalyst with copper supported by a zinc aluminum composite oxide carrier can keep high activity even when exposed to oxygen gas at high temperatures (Patent Reference 4).
There are given various proposals for improved methods as in the above, but for use for fuel cells that are frequently started and stopped, they are not still on a satisfactory durability level.
Patent Reference 1: JP-A 2003-236382
Patent Reference 2: JP-A 2004-122063
Patent Reference 3: JP-T 2005-520689
Patent Reference 4: JP-A 2003-275590